Supermassive black holes - with masses of millions to billions of times that of the Sun - reside in the nuclei of galaxies. While black holes are not directly visible, surrounding material becomes extremely luminous before being accreted, creating telltale signatures of black hole activity. In turn, the amount of activity tells us about black hole growth, and about energy injection back into the host galaxies. This Active Galactic Nuclei (AGN) feedback is thought to play a role in regulating the rate at which galaxies form new stars, thereby directly affecting their evolution across cosmic time. I will present two different approaches -- a detailed case study, and a large population study -- to shed light on the growth of black holes, and their influence on their host galaxies. In particular, I will introduce revised observational methods to identify AGN from spectroscopy and photometry, which yield a more complete census. Our results imply a stronger connection with the gas reservoirs (and/or potential well) of galaxies than previously thought. I will also highlight findings from a multi-scale analysis of gas ionization and dynamics thanks to 3D spectroscopy with the VLT/MUSE instrument. I will conclude with exciting new opportunities from large galaxy survey experiments such as DESI and Euclid, which will yield unprecedented samples of several million galaxy and quasar spectra.